There are many oxides of uranium, including UO.sub.2, UO.sub.3, U.sub.3 O.sub.8, and U.sub.4 O.sub.9. When uranium dioxide powder is manufactured for use as fuel pellets in nuclear reactors, it can sometimes spontaneously oxidize to form an oxide containing more oxygen. This process, known as "burnback," liberates an undesirable amount of heat and reduces the ceramic activity of the powder, rendering it unusable without further treatment.
The likelihood that a powder will spontaneously oxidize depends upon the activity of a powder, which is primarily a function of its surface area and the oxygen-to-uranium (O/U) atomic ratio of the powder. A powder having a fine particle size (and therefore a large surface area), or having a low O/U ratio is more likely to spontaneously oxidize that is a coarser powder, or a powder with a higher O/U ratio.
Methods have been proposed for stabilizing uranium dioxide powder by partially reoxidizing the powder under controlled conditions to a slightly higher O/U ratio. These methods involve exposing the powder to a small amount of oxygen so that those particles that are most sensitive to spontaneous oxidation are oxidized to a more stable state. In some of these methods, the powder is fluidized in the presence of diluted oxygen. While this will stabilize the powder, it creates additional problems in that fluidized uranium dioxide powder is difficult to handle and must be carefully filtered to prevent the very fine particles from escaping into the atmosphere.
Another method of partially oxidizing uranium oxide powders is described by R. Lloyd in a paper entitled "Uranium 233 Purification and Conversion to Stabilized Ceramic Grade Urania for LWBR Fuel Fabrication (LWBR Development Program)," dated October, 1980, and available from the National Technical Information Service of the U.S. Department of Commerce. In Dr. Lloyd's processes, water is added to the powder to occpy the oxygen absorption sites on the particles. While this stabilizes the powder, it is unsuitable for use with enriched uranium where the uranium must be handled in bulk because water acts as a neutron moderator, generating large numbers of thermal neutrons which can cause the powder to become critical.